The transforming proteins of Harvey (Ha) and Kirsten (Ki) murine sarcoma viruses, Ha-ras and Ki-ras, are 21,000 dalton polypeptides. The nucleotide sequence of the Ha murine sarcoma virus gene and its two rat cellular homologues, c-ras I and c-ras II, have been determined. The sequence corresponding to the N-terminal region of the human oncogene from EJ carcinoma cell line and its proto-oncogene (human c-Ha-ras) have also been determined and the crucial difference between these genes has been located within the 12th amino acid codon. A rapid method has been developed to analyze mutations at the 12th codon in different human tumors.